Gear train units having a plurality of planetary gear train stages, which are connected one after the other, are employed when extremely high torques are required at low rotary speeds.
In case the gear reduction of the planetary gear train stages, which are connected one after the other, is required only temporarily and in normal operation a smaller or no gear reduction is required, which e.g. is the case with creep speed gear units for mechanical presses, it is desirable that the gear unit in addition provides the gear ratio required for normal operation and can be shifted without using any tools back and forth between the two gear ratios, for allowing an easy switching between normal operation and creep speed operation.
Often, it is not only important that the gear unit is inexpensive and sturdy, but that it is also of as compact as possible design in order to be able to be integrated into existing mechanical parts, e.g. into the belt pulley of the belt drive of the press.
Hence, there arises the objective to provide a shiftable gear train unit having a plurality of planetary gear train stages, which are connected one after the other, which is inexpensive, sturdy and as compact as possible.